The present invention relates to a transparent conductive ink for forming a transparent conductive film by gravure printing on surface of multicolored patterns printed by offset printing on a base material such as paper. Printed materials with conductivity provided by the transparent conductive ink of the present invention are preferably used for illustrated books and toys for educational purpose.
Teaching materials printed using conductive ink have been known in the past. Such printed teaching material comprises, for example, as shown in a set of questions for learning (FIG. 1) disclosed in Japanese Utility Model Publication Laid-Open 48-20540, a set of questions (question items) 2 printed using non-conductive ink on a base material 1 and a check mark 3 printed with conductive ink (or non-conductive ink). In this case, two or more answers are printed for a question, and check marks are printed to match the answers, and only the check mark for the correct answer is printed with the conductive ink. A pupil or a student can confirm correct answer or incorrect answer by pressing the check marks using a light pen (not shown) for checking, which is lighted up only when electric conductivity is given by a conductive ink layer. Therefore, the teaching material using this type of conductive ink is advantageous in that self-check can be performed quickly by a pupil or a student, and that the teaching material is available for repeated use.
In the conductive ink used in this type of printed materials, carbon powder or silver powder is used as a conductive component. When it is used only for printing of check marks in the teaching materials as described above, there is no problem. However, on the illustrated books for children for education purpose which must be printed with diverse picture patterns, it is not always easy to print multicolored picture patterns or images freely with the conductive ink. Because the conductive ink is not transparent, even when printing pattern with this ink is overlapped on multicolor picture pattern or image, it is not possible to obtain the picture pattern and the image in the color as desired.
In the Japanese Patent Publication Laid-Open 7-57545, a transparent conductive printed material is disclosed, which comprises a conductive sector having an ink layer, a barrier layer and a transparent conductive ink layer of characters or picture patterns deposited sequentially on a printing base material in this order from the side of the printing base material. Then, this is placed on the surface of multicolored picture patterns produced by offset printing, and a transparent conductive film is formed by gravure printing using the transparent conductive ink.
The standard operating condition of this type of gravure printing is characterized by high-speed printing (30 to 120 sheets per minute) and low temperature short-time drying (30 to 60 seconds at 60.degree. C.). The printed materials are stacked (5000-10000 sheets; 1 meter in height; pressure at the bottom: 10 kg/cm.sup.2) before the bookbinding is performed. In this respect, the transparent conductive ink used for gravure printing must meet the requirements such as suitability for gravure printing under the above operating condition, high blocking resistance, stable attainment of conductivity and maintenance of freshness of colors in picture patterns and images.
The ink for gravure printing must satisfy the requirements such as limited drying condition, less residual solvent in the dried printed material and quick decrease of residual solvents. For this reason, the solvent contains toluene as major component, and a resin soluble in toluene is generally selected as a binder. Chlorinated rubber resin contains chlorine, which may cause pollution problem when it is incinerated. Alkyd resin has poor drying property and pyroxylin must be simultaneously used, and ketone type or ester type solvent to dissolve pyroxylin must be contained as essential component. If the content of ester type or ketone type solvent is decreased in the composition of the solvent during printing, there is a problem that pyroxylin may be deposited.
The above patent publication discloses a transparent conductive ink having the following composition: polyester resin 10.5%, mixture of tin oxide and antimony pentoxide 24.5%, and mixed solvent of toluene and methylisoburylketone (50%/50% mixture) 65%. This transparent conductive ink is also disadvantageous in that drying property is poor, solvents are very likely to remain in the dried ink, and viscosity varies widely due to change in the solvent composition when the mixed solvent is used.
It is an object of the present invention to provide a transparent conductive ink used when a transparent conductive film is formed by gravure printing on the surface of multicolor picture patterns or images produced by offset printing, whereby the conductive ink has property suitable for gravure printing, high blocking resistance and stable attainment of conductive property, and provides freshness of is color in picture patterns and images.